Both Emp and Max always on.
Its like Power Attack for melee. Do I want to damage or just tickle.
Mana efficiency comes from AoE and "one turn advantage". I don't want to waste mana on a single CR0.25 kobold, I want to nuke a small armada with a single spell. And I don't want to recast it. Maybe "recast" is mana efficient, but its "time". I mean I could drop dead in that time or I'll take damage and then have to heal myself. Heal is also mana.